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The physiological theory

This theory could explain why we tend to yawn when there is a group of people in a room, but it doesn't shine any light on why we also sometimes yawn when we exercise.

And a neuroscientist at the University of Maryland, Robert Provine, added more doubt to the theory. His research found that giving people more oxygen didn't stop yawning, and decreasing the amount of carbon dioxide also made little difference.

The theory of evolution

Some experts believe the humble yawn is a throwback to our ancient ancestors, who used to show their teeth to intimidate others.

Others say yawning was developed in caveman times as a subtle signal to switch activities.

The latest theory is that we yawn to help keep our brain cool and alert. Photo / Getty